How do you invoke schemagen in Java 11?

The issue seems to be known with the jaxb-*:2.3.0 version - #jaxb-v2/issues/1168. Additionally, this would be resolved with a future release as marked in the known issues of jaxb running over java-9.

You can resolve this following the comment -

Please try 2.4.0-b180725.0644 - this is JPMS modularized RI working on JDKs with java.xml.bind module (9,10) and those without it (11-ea)

or try downloading the binary distribution from the same link.

Schemagen and xjc shell scripts are only put into binary distribution in ./bin directory.

For build tools there are plugins out there (Maven / Gradle) which does invoke schemagen and xjc APIs, providing user with simple configuration.

Your attempt to invoke com.sun.tools.jxc.SchemaGeneratorFacade manually is also correct, here is similar example for Maven. However you are probably putting 2.3.0 on module path, which has a split package problem. Putting on classpath will resolve the issue for 2.3.0. Next release of JAXB will be JPMS ready and have module descriptors declared. You can try the beta build (2.4.0-b180725.0644), here is a correct set of dependencies:

<dependency>
  <groupId>org.glassfish.jaxb</groupId>
  <artifactId>jaxb-runtime</artifactId> <!--jaxb runtime-->
</dependency>

<dependency>
  <groupId>org.glassfish.jaxb</groupId>
  <artifactId>jaxb-xjc</artifactId> <!--java generation-->
</dependency>

<dependency>
  <groupId>org.glassfish.jaxb</groupId>
  <artifactId>jaxb-jxc</artifactId> <!--schema generation-->
</dependency>